B and T lymphocytes: origins, development, and surface receptors

B and T. Lymphocytes

The Biology of B. And T. Lymphocytes and the Reactions between Them

The Biology of B. Lymphocytes

B lymphocytes originated in 1960s and 1970s era through experiments conducted in animal models, clinical evaluation of patients having immune system diseases, and the nascent technology of cell surface molecule characterization. In fact, the origin of B. And T. lymphocytes took place simultaneously. The differentiation of the haematopoietic stem cells gives birth to the formation of common lymphoid progenitors, which actually generate B. lymphocytes. They are generated and developed in yolk sac, fetal liver, and the adult liver present in the body (Austyn & Wood, 1994). B lymphocytes are present in areas that come in close contact with foreign substances. They act as defensive mechanism against invading microorganisms, viruses and parasites and play a vital role in humoral immune response. Since these cells originate in the Bone marrow, they are referred as B-cells. B lymphocytes are further differentiated into antibody forming cells which are called plasma cells. These produce glycoproteins which are called immunoglobulins (antibodies).

B-cell development occurs in three main phases. Mature immunocompetent B cells are formed during the initial, antigen independent phase, which can easily bind to a unique antigen. B cells further form immunocompetent naive mature follicular B cells in the bone marrow and spleen during the antigen independent maturation phase. Co-stimulation and antigen binding activates B cells leading towards B cell development in the antigen dependent phase. During this stage, B lymphocytes differentiate into either memory B cells or terminal, antibody-secreting plasma cells. This also involves proliferation of activated B cell, maturation of antibody affinity, and antibody class switching within the secondary lymphoid tissues (Leiber & Tedder, 2008).

A typical B cell antibody molecule is Y shaped, where tips of the Y have identical antigen binding sites and the tail of Y caters the cell-surface receptors. The molecule consists of four polypeptide chains, two identical light chains and two identical heavy chains. Each light and heavy chain is composed of a number of Ig domains -- ? sheet structures containing about 110 amino acids. Heavy chains determine the biological properties of class of antibody which determines what proteins will bind with the antibody.

The Biology of T. Lymphocytes

Thymocytes, T-Cells or Thymus cells are white blood cells which aid in fighting against diseases and infections. They have the distinguishing feature of having a special receptor on their cell surface called T-cell receptors which cater the identification of specific antigens that enter the body. These cells are produced in the bone marrow but mature in the thymus gland. There are several types of T-cells, each one having its own function. Some of them include cytotoxic T-cells, Helper T-cells, memory T-cells, regulatory T-cells, natural killer (NK) T-cells, and Gamma/delta (Y?) T-cells (Austyn & Wood, 1994).

When T cells are present in the bone marrow, they do not posses receptors on their surfaces yet because they do not express CD4 or CD8 glycoproteins (carbohydrate and protein molecules located on the surface of T-cells). Therefore, they are considered double-negative cells (Cd4- Cd8-). T cells become double positive when they become mature. Then MHC molecules help T-cells detect host cells that have been invaded by infectious organisms and then bind to the MCH peptide. T-cells which bind successfully with MCH peptide survive only, while the rest die out. Activated T-cells become helper T-cells, also called CD4 T-cells or effector cells. These cells divide rapidly and secrete proteins called cytokines, which trigger immune cells to engulf the antigen (Beltman et. al, 2007). Mature T-cells are presented with a self-antigen which combines with MHC molecules on antigen-presenting cells (APCs) like B-cells, dendritic cells, and macrophages. The activation of T cells require two signals, first identifies the foreign substance while the other occurs after binding with the invader. T cells reacting too strongly with antigen receive a death signal from the Antigen-Presenting Cells (APC).

Importance of Cytokines and their Receptors in Immune System Development and Responses

Cytokines are a type of proteins playing the role of a messenger between cells and regulating several inflammatory responses. They act as intercellular mediators and are clinically important as biological response modifiers. Cytokines play the role of hormones and are also referred as the hormones of the immune system. Hormones cater specific organs, while cytokines have their effect on different cell populations and tissues. Cytokines induce macrophage and NK cell activation which generates inflammatory and chemotaxis processes and are thus involved in innate immune response. They act on T. And B. lymphocytes which cater the communication among different cell populations and is therefore vital adaptive immune response. They actually cater the development, maturation, localization, interactions, activation and life span of immune cells. Cytokines chemically attracts specific cell types. Cells release cytokines at the site of infection or injury and also call other immune cells to the region to help fight or combat the invader. They are pivotal to inflammation and could be added to drugs for regulating immune responses.

Moreover, cytokines activate several cytosolic signal transduction proteins. These comprise of protein tyrosine kinases and phosphatases which variety of cellular functions, including proliferation and differentiation. These receptors have paved the way for the control of normal hematopoiesis. Interferon is a popular cytokine which helps in the elimination of pathogens residing in intra-cellular vesicular compartments and also enhances the microbicidal function of macrophages through formation of nitric oxide and reactive oxygen intermediates. Cytokine base immunotherapy is considered to be instrumental in treating cancer. Th2 immune responses are linked with allergic diseases. The alteration of cytokine producing profile of Th2 cells by inducing Th1 responses could be beneficial in protecting against asthma and allergy (House & Descotes, 2010).